Dusting of rubberlike materials with cellulose



Patented Mar. 30, 1954 DUSTING OF RUBBERLIKE MATERIALS WITH CELLULOSEJohn R. Gilcrease, Baton Rouge, La., assignor to Standard OilDevelopment Company, a corporation of Delaware No Drawing. ApplicationDecember 22, 1950, Serial No. 202,419

7 Claims. (Cl. 260-174) This invention relates to rubbery materials andrelates particularly to a means for coating tacky materials such asbutadiene-nitrile rubber crumbs so as to prevent the individual rubberparticles from sticking together or coalescing.

It is well known that most uncured or raw stocks of rubber-likematerials, because of their appreciable tack, require surface dustingwith various powders to prevent adhesion to containers or coalescence ofseparate rubber-like particles during storage and the like. Especiallyin the case of rubber-like materials which are in the form of smallpellets or crumbs, such as are increasingly in demand for compoundingwith resins, molding or for the formation of liquid solutions, theproblem of preventing coalescence during storage has been gaining inimportance.

Common dusting powders heretofore used for this purpose have includedprincipally talc, soapstone, zinc stearate and similar inorganicsubstances and organic metal salts. However, the use of all thesematerials has been only moderately successful since all such materialsgradually tend to sink into the dusted surface of the rubber particles,or some become at least partially dissolved therein. Hence theeffectiveness of the powders disappears with time, particularly whenrelatively high ambient temperatures lower the viscosity of therubber-like material. Moreover, some of the prior art dusting materials,when used, in concentrations required to give reasonably effectiveprotection, have the disadvantage of interfering with the eventual cureoi the dusted rubber and often they aifect adversely the electrical andother properties of the final vulcanizate.

A surprisingly effective dusting material has now been discovered incellulose, particularly in fibrous wood cellulose of a certain kinddescribed subsequently.

This invention is particularly applicable to nitrile rubber, that is, toemulsion copolymers of a major proportion of a conjugated Ct to C6diolefin, e. g. 50 to 85 parts of butadiene-l,3, isoprene or2,3-dimethylbutadiene-1,3 and a minor proportion of a nitrile, e. g. 50to parts of acrylonitrile, alpha-methacrylonitrile oralphachloracrylonitrile. It is of special value in conjunction withcrumbs of nitrile rubber having a nitrile content between about and andhaving a Mooney viscosity between about to 90, which type of polymershave a very pronounced cohesion tendency because of the relatively softnature. However, the invention ofiers some degree. of improvement alsowith harder nitrile rubbers, as well as with natural rubber. GR-S typerubbers (emulsion copolymers of to 85 parts of a diolefin such asbutadiene or isoprene with 40 to 15 parts of styrene), polychloroprenerubbers, polysulfide type rubbers, GR-I type rubbers (copolymers of amajor proportion of isobutylene and a minor proportion of isoprene orbutadiene), as well as with rubberlike polyethylene, polyisobutylenehaving a Staudinger molecular weight between about 60,000 and 200,000 ormore, styrene-isobutylene polymers such as are described in U. S. Patent2,274,749 and so forth.

The dusting material of the invention is an essentially pure, powderedcellulose fiber having an average particle size of 30 to 200 microns,preferably about 100 to 165 microns. When subjected to a screenanalysis, essentially all of the preferred cellulose material(Solka-Floc) passes through 35 mesh, to passes through mesh and aboutv30 to 50% passes through 200 mesh (Tyler scale). The material is aprimary pure cellulose product obtained from wood by the sulfite and/orcaustic process which separates lignin and other substances from thedesired cellulosic fiber. The pure cellulosic product used in theinvention has no direct relation to and is not interchangeable with woodfloor. The desired powdered fiber is essentially white, may contain upto about 5 or 10% of moisture, has a specific gravity of 1.58 and a bulkdensity or about 2 to '7 cc./gram, preferably about 4 to 'I cc./gram.

Other materials which are similarly useful, though not quite soeffective, are other cellulosic fibers having approximately the samedimensional characteristics and specific gravity between about 1.3 and2.0, such as powdered cotton fibers or even rayon and other regeneratedcellulose fibers. Ordinary wood flour, even if its average particle sizeis within the limits above specified, has been found ineffective forpurposes of this invention, presumably because its irregular, more orless round shape makes occlusion in the rubber relatively easy. Incontrast, fibrous materials par ticularly suited for the presentinvention have an elongated shape characterized by a substantiallygreater length than diameter, so that even if such a fibrous particlebecomes partially embedded in the rubber a substantial part of theparticle will remain above the surface and protect the latter againstcoalescence. Furthermore, ordinary wood flour is not nearly so efiectivebecause of its lignin content. which materially affects itsanti-coalescent effectiveness.

The following examples illustrate the present invention, though it willbe understood that the latter is not limited thereto. All quantitiesexpressed therein, as well as in the rest of the specification andclaims, are expressed on a weight basis unless otherwise indicated.

Example 1 A commercial-grade, rubber-like nitrile copolymer of about 74%combined butadiene and 26% combined acrylonitrile-having a- Mooneyviscosity of about 60 was prepared by conventional emulsion techniquecarrying the polymerization to about 75% conversion. The latex resultingfrom this polymerization was shortstopped in a conventional manner toprevent further polymerization, an antioxidant such as 2,6-di-tert-butyl-l-methyl phenol was added thereto, and stripped ofunreacted monomers in the usual manner. The stripped latex waseoagulated with a concentrated sodium chloride solution, and theparticles of coagulated polymer were screened from the aqueous liquid,passed through a roller to remove excess water and through a hammer millwhere it was restored to its original granular coagulate size, and driedin ahot air oven at a temperature of about C. to 90 0., preferably about70 C. The dried coagulate was in the form of an agglomerated sheet andthis I Example 2 The comparative efiectiveness of the novel fibrouscellulose and conventional dusting agents is summarized in the followingdata obtained on two commercial grades of butadiene-acrylonitrile rubbercrumb having a Mooney viscosity value of 90, a combined nitrile contentof 35% and 26% respectively, and an average particle diameter of about 2to 4 mm. The dry rubber crumbs were coated with the type and amount ofdusting agent indicated in the table below, the dusting operation havingbeen carried out by vigorous shaking of the rubber crumb and dustingagent in a closed one-quart container for approximately seconds.

TABLE Concentration of Coating Agent and Conditions Rubber 0 t A tStorage of Sample After Storage 3 0th 0 t oa ing gen ime, er ommen sGrade Months None l N. G N. G N. G. N. G N. G N. G. Vinyllte Powder 8 N.G. S S S Marviuol Powder a... 8 N. G. S S S Cellulose Fiber 6 8 S S E EE E Calcium Silicate 8 E E E Retards mercaptobenzothiazole accelerators.None 1 N. G N. G N. G. N. G N. G N. G Wood Flour 5 1 N. G N. G. N. G S SMarvinol Powder l N. G N. G. S E E Cellulose Fiber 6 8 S S E E E lParacril NS 90 (copolymer of 65% butadiene35% ri rylonitrilc; Mooneyviscosity 90).

2 Paracril 26 NS 90 (copolymer of 74% butadiene26% rylonitrile; Mooneyviscosity 90).

3 Code: E-exccllent (crumbs remained free flowing alter storage);Ssatisiactory (crumbs were loosely bound alter storage); N. G.no good(crumbs coalesced into a solid mass during storage).

4 Vinylite VYHH, powdered vinyl chloride-acetate copolymer of 85% vinylchloride; ave. M. W. 10,000.

5 Marvinol VII-10, vinyl chloride polymer, size 100% through 60 mesh,sp. gr. 1.4.

6 Solka-Fioc SWiOA, pure wood cellulose fiber, ave. particle size 165microns, size 77% through 100 mesh and through 200 mesh, sp. gr. 1.58;bulk 6.8 ccJgram.

7 Silene EF, hydrated precipitated calcium silicate, pariicle size 0.2to 0.3 micron, sp. gt. 2.1.

! Masonoid, wood flour. size 105% through 80 mesh.

was returned once more to the hammer mill where it was again shreddedapproximately to its original crumb size, corresponding to particlediameters ranging from about g to 1 inch, or mostly from about t; toinch. Some of the resulting product, known in the art as dry crumb, wasfinally pressed into lb. bales for packaging. Both the loose crumb andthe pressed bales formed a solid mass when stored for days in cardboardcontainers under simulated storage conditions, i. e. at a temperaturewhich varied during the course of the test between F. and 97 F. Anotherhammer mill operation was necessary to produce crumbs from the storedproduct. This illustrates the impossibility of preserving nitrile rubberin loose crumb form for any extended period under ordinary storageconditions.

On the other hand, samples of the above-described dry nitrile rubbercrumbs were mixed, respectively, with 1%, 2%, 4%, 6% and 10% (based on arubber) of the preferred powdered pure wood cellulose fiber describedearlier herein by shaking the nitrile rubber crumbs and the statedamounts of cellulose in a closed container, until The data show that inthe absence of any dusting agent the rubber crumbs coalesce into a solidmass within one month. And even with common dusting agents such aspowdered vinyl chloride polymers or wood flour, the crumbs coalesceexcessively unless the dusting agent is used in relatively largeamounts, e. g., at least 5 to 7% by weight of the rubber, and even thensome bindin of the particles will occur.

On the other hand, calcium silicate proved very eilective in preventingcoalescence of the individual particles during storage but, as is wellknown, its presence exerts an undesirable retarding effect on theeventual cure of the rubber, especially when an accelerator of themercaptobenzothiazole type is used in the compounding recipe. Aluminumstearate, not shown here, is likewise a fairly effective dusting agentwhen used in concentrations of at least 4%, but it too retards the cureof the rubber. Conversely, still other dusting agents such as zincstearate or calcium carbonate, while fairly effective in preventingcoalescence when used in adequate concentration, may also beobjectionable since they tend to Shorten the curing time andoccasionally may even lead to scorching of the compounded stock duringpreliminary processing,

In contrast to all of the agents discussed above, the powdered cellulosefiber has proved to be an exceptionally effective dusting agent even atrelatively low concentrations, and being entirely inert and withouteffect on the eventual cure, it permits anentirely free cboiceof,vulcanization formula. Specifically, the tests show that as little asabout 1.0% of cellulose fiber dust per weight of polymer is adequate forreducing the coalescence of the polymer particles to a valuesuificiently low for many purposes; and in concentrations of about 4% toof the cellulose fiber dust any tendency to coalesce between individualpolymer particles is substantially completely eliminated.

The rubber particles coated in accordance with the teaching of thepresent invention are highly useful for blend ng with other materials,and in general the small amount of cellulose dust has no noticeableeffect on the phys cal properties or appearance of the blend. Forinstance, the coated nitrile rubber crumbs can be readily blended withpolyvinyl chloride-acetate resins in the usual manner. Also, due totheir relatively small size, the discrete nitrile rubber crumbsobtainable by means of the present invention are readily dissolved insolvents such as methyl ethyl ketone for subsequent use as a coatin or acement. Here again, the presence of the cellulose dust is usually unobectionable. In special cases where the elimination of all foreign matteris essential because of product clarity considerations, and hencefiltration of the polymer solutions is required whether or not a dustingagent was used, the cellulose dust of the invention may actually act asa filter aid.

While the invention has been illustrated in terms of certain specificembodiments, it will be understood that the described procedures may bemodified and varied to give best results for any given set ofconditions. For instance, the optimum proportion of cellulose dust maybe found to difier somewhat from case to case depending on particle sizeand viscosity or tackiness of the rubbery polymer and also depending onthe particle size of the cellulose dust. Also, while the given examplesdescribe dusting of polymer particles by shaking in a closed containerafter the particles had been dried, it is equally feasible to carry outthe dusting step otherwise. as by placing the dusting agent, by means ofa vibrating proportioner or the like, in a screw conveyor such as isused in industry for carrying the rubber from one manufacturing stage toanother, in which case proper dusting may be obtained in the conveyoritself when the rubber is present there in the desired final size, orthe dusting may take place in a shredder if the dusting agent is .1

introduced into a conveyor carrying the coalesced rubber to a hammermill or the like. Al-

ternatively, it may be considered. preferable to mix the cellulosedusting a ent with the polymer particles in a rotary drier while the wetpolymer particles are being dried; or the mixing may be done directlyafter coagulation of the latex and prior to drying of the coagulatedparticles, which procedure has the additional advantage of great- 1yreducing the usual agglomeration of the polymer particles in the drier.Another possible procedure involves adding the cellulose dust directlyto the latex even prior to coagulation, though it will be understoodthat in such an instance somewhat larger amounts of dusting agent arerequiredzducto thefact that some, of the cellulose dust becomes occludedin the polymer particles during coagulation andto that exent becomesunavailable for protecting the surfaces of the resulting polymerparticles against agglomeration or coalescence.

Still other modifications or variations not specifically suggestedherein may occur to persons skilled in the art without constituting a.departure from the scope and spirit of the invention claimed below.

The invention claimed is:

1. As an article of manufacture, a rubbery butadiene emulsion polymer incrumb form, said crumbs having a particle size of about to 1 inchaverage diameter and being coated with at least 1.0% based on the weightof polymer of a dusting agent consisting of substantially pure fibrouswood cellulose reduced in size so that at least 65% of the groundparticles pass through a ion-mesh screen.

2. An article of manufacture according to claim 1 wherein the rubberycrumbs consist essentially of a copolymer of 50 to 85 percent ofbutadiene-1,3 and 50 to 15 percent of acrylonitrile having a Mooneyviscosity between 40 and 90.

3. An article of manufacture according to claim 2 wherein the dustingagent is a substantially pure, lignin-free, fibrous wood cellulosepowder having a specific gravity of about 1.58, a bull: density betweenabout 4 and 7 cc./gram and reduced in size so that 65 to 85% thereofpasses through a IOU-mesh screen and to 50% passes through a ZOO-meshscreen.

4. A process which comprises coating of the surface of a mass ofdiscrete particles of a rubbery copolymer of about 50 to 85% of aconjugated C4 to C6 diolefin and 50 to 15% of acrylonitrile having aMooney viscosity between 40 and 90 with 1.5 to 10% based on thecopolymer of substantially pure fibrous cellulose powder having anaverage particle size between about 30 and 200 microns.

5. In a process of making rubber crumb, the improvement which comprisesshaking 100 parts of a mass of discrete particles of a dry rubberycopolymer of about 25 to acrylonitrile and '75 to 65% of butadiene-1,3having a Mooney viscosity between 40 and 90, said rubbery particleshaving a diameter in the range between and 1 inch, and 1.0 to 10% basedon the rubber of a substantially pure fibrous wood cellulose powderhaving an average particle size between 100 and 165 microns, the shakingbeing continued until said cellulose powder becomes distributed aroundthe rubbery particles.

6. In a process of making rubber crumb, the improvement which comprisescoating the sur face of freshly coagulated wet particles of a. rubberycopolymer of to butadiene and 35 to 25% acrylonitrile having a Mooneyviscosity between 40 and 90, said particles having a diameter in therange between and inch, and 4 to 10% based on the rubber of asubstantially pure fibrous wood cellulose powder having an averageparticle size between and microns until a protective coating of saidcellulose powder is distributed around the rubber particles, andthereafter drying the coated particles in an air drying zone maintainedat a temperature between about 50 and 90 C.

I. A rubbery butadiene emulsion polymer in crumb form having the surfaceof discrete particles thereof coated with 1.0%, based on the 7 polymer,of substantially pure fibrous wood cellulose.

JOHN R. GILCREASE.

; References Cited in the file of this patent UNITED STATES PATENTS 8FOREIGN PATENTS Number Country Date 459,736 Canada Sept. 13, 1949 OTHERREFERENCES Goodloe Rubber Age, v01. 61, pages 697-703, September 1947.

Compounding Ingredients for Rubber, Second Edition (1947), page 353.

1. AS AN ARTICLE OF MANUFACTURE, A RUBBERY BUTADINE EMULSION POLYMER INCRUMB FORM, SAID CRUMBS HAVING A PARTICLE SIZE OF ABOUT 1/32 TO 1 INCHAVERAGE DIAMETER AND BEING COATED WITH AT LEAST 1.0% BASED ON THE WEIGHTOF POLYMER OF A DUSTING AGENT ON THE WEIGHT OF POLYMER OF A FIBROUS WOODCELLULOSE REDUCED IN SIZE SO THAT AT LEAST 65% OF THE GROUND PARTICLESPASS THROUGH A 100-MESH SCREEN.